Method for producing a lignite product

ABSTRACT

A method for producing a lignite product comprising the steps of (a) digging lignite from a lignite seam, (b) grinding the lignite, (c) forming a slurry comprising the lignite, (d) transporting the slurry to a locus for drying, and (e) drying the slurry to form a lignite product, wherein steps (a) to (c) are carried out using a mobile device.

FIELD OF THE INVENTION

The invention relates to a novel method of producing a value-addedlignite product. In particular, the present invention relates to aprocess that provides a comparatively low moisture content product thathas improved handling and utilisation compared with unprocessed lignite.

BACKGROUND OF THE INVENTION

In this specification where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date, publicly available, known to thepublic, part of common general knowledge; or known to be relevant to anattempt to solve any problem with which this specification is concerned.

Coal is a fossil fuel formed in swamp environments from the remains oforganic matter such as plants that have been preserved (at least inpart) by immersion in water and mud. Coal is a non-renewable energysource because it takes millions of years to create. Coal is classifiedinto four main types, or ranks (lignite, sub bituminous, bituminous,anthracite), depending on the amounts and types of carbon it containsand on the amount of heat energy it can produce. The rank of a depositof coal depends on the pressure and heat acting on the organic matter asit sank deeper and deeper in the earth over millions of years. For themost part, the higher ranks of coal contain more heat-producing energy.

World coal consumption is about 5.3 billion tons annually. Coal isprimarily used as a solid fuel to produce electricity and heat throughcombustion.

Bituminous coal contains 45 to 86 wt % carbon, and has two to threetimes the heating value of the lowest rank of coal, which is known aslignite or ‘brown coal’. Bituminous coal was formed under high heat andpressure and is typically between 100 and 300 million years old.Bituminous coal is used to generate electricity and is an important fueland raw material for the steel and iron industries

Anthracite contains 86 to 97 wt % carbon, and has a heating valueslightly lower than bituminous coal. Sub bituminous coal typicallycontains 35 to 45 wt % carbon. Most sub bituminous coal is at least 100million years old.

Lignite is the lowest rank of coal with the lowest energy content andused almost exclusively as fuel for steam-electric power generation. Itcontains 25 to 35 wt % carbon. Lignite deposits tend to be relativelyyoung (around 15 and up to 50 million years old) and have not beensubjected to extreme heat or pressure. Lignite is ‘crumbly’ and has ahigh inherent moisture content, sometimes as high as 70 wt %, bulkdensity as low as 1.07 gcm⁻³. In many instances lignite has a very highash content compared with bituminous coal, however Latrobe Valleylignite (deposits in the Gippsland region of Victoria, Australia) hascomparatively low ash content. It is also a heterogeneous mixture ofcompounds of various structural formulae.

Large reserves of lignite are located in Germany, Russia, the USA,Australia, China, and Eastern Europe. Because of its low energy densityand therefore energy potential, lignite is inefficient to transport andis not traded extensively on the world market compared with higher coalgrades. It is often burned in power stations constructed very close toopen cut brown coal mines, such as the power stations located in theLatrobe Valley, Victoria and Monticello, Tex.

Lignite coal is often made into ‘briquettes’. Briquette manufacturetypically involves drying lignite to about 12 to 18 wt % moisture,optionally adding binders, then ram-extruding the dried mixture.Briquettes are useful because they render the lignite easier totransport and handle and are primarily used in household and industry.

In the past attempts have been made to create other good quality,readily transportable lignite products. For example, Australian patentapplication 36096/95 relates to a batch process for manufacturing a lowwater content lignite product, the process comprising pressing granularlignite on a flat bed, steam heating the pressed coal, then imposingfurther pressurisation to expel water. The product is apparently a drygranular or powdery mass, which would then be expected, due to its drynature and high fines content, to require special safe handling and safestorage facilities to prevent spontaneous combustion or explosion, orrehydration of the product. The lignite processing would appear to becarried out separately from the winning, transportation, handling andstorage of the coal.

Australian patent application 37292/89 relates to a process forimproving the quality of lignite. The process steps include combining itwith additives, shearing and attrition, followed by extrusion and dryingto create densified coal pellets. The characteristics of the pelletisedproduct depend upon the additives used during the shearing and attritionsteps, however the pellets would need to be sufficiently robust to bestored outdoors and subjected to the weather, to avoid high cost storageand handling equipment. The nature of the process requires separate coalwinning and transportation to the process plant which may well be awayfrom the coal winning surface.

Australian patent application 63621/86 relates to a process fordewatering lignite to produce a low water content product. The processcomprises steam dewatering using a plurality of autoclaves in sequence,in a batch process. This necessarily requires the use of a steam raisingplant. Again the question of handling, storage and transport arises.

Australian patent application 68491/74 relates to a process forupgrading lignite. The process steps include grinding the low rankingfuel to a fineness that will pass through a 300 BSS mesh (48 μm),sufficient to destroy the cellular structure of the coal, forming aslurry, and then drying the slurry.

International patent application WO/2007/066191 relates to a method ofimproving the physical properties of solid fuels such as lignite. Theprocess principally consists of introducing lignite to a hot oil bath tochange the handling and storage characteristics of the lignite.International patent application WO/1987/005891 relates to a similarprocess comprising placing crushed coal in hot oil to dry the coal andchange its handling and storage characteristics.

International patent application WO/2006/119044 relates to a method ofchanging the bulk characteristics of lignite using high pressurecompaction and comminution processes to eliminate voids in the bulkproduct.

International patent application WO/2006/043162 relates to a device forprocessing lignite to form an extruded ‘worm like’ or pelletised fuel.The device comprises a crushing element and mills with grinding bodiesincluding a low wear grinding device suitable for lignite, mixers,extruders and a pressing worm provided with the crushing elements.

International patent application WO/2005/003255 relates to a method forupgrading low rank coal stocks using acidic aqueous waste mixed withcoal to remove high ash material. Removal of ash is useful in raisingthe value of the fuel, but acid treatment of lignite coal typicallycreates a light, fluffy coal product that is unlikely to create aproduct having desirable hard, dust free and good weatheringcharacteristics.

International patent application WO/2001/054819 is a further process forupgrading low rank coal by formation of a plastic mass under the shearaction of rollable surfaces. In order to achieve the desired degree ofcell or pore destruction, multiple crushing passes may be required.

Many of the processes disclosed in prior patents relate to dryinglignite. However the products of these processes typically consist of adry powder which is potentially hazardous due to the risk of powderexplosion, difficult to handle because of the high fines content andraises occupational health issues for workers who may inhale the fines.Accordingly, the products typically require special handling and costlystorage facilities. For example, International patent applicationWO/1997/035944 relates to a process for comminuting and drying ligniteusing multiple comminution steps and a fluidised bed drying process.International patent application WO/1997/031082 relates to a process forreducing the water content of lignite by exposing the lignite to thermalenergy and pressure on a flat bed. International patent applicationWO/1996/010064 relates to a method for reducing the water content oflignite by the application of heat and pressure to granular brown coalspread out in a bed. International patent application WO/1992/014801relates to a process for drying hydrous solid fuels, in particularlignite using a drying container linked into the steam circuit of asteam generating plant.

The aforementioned processes are likely carried out at a purpose-builtfacility remote from the seam from which the lignite is won,necessitating transportation and storage of the coal at a processinglocation. None of the aforementioned processes appear to be readilyadapted for mobile performance or location at the coal digging face.Several require expensive fixed plant or steam/heat generationfacilities.

Furthermore, typically the dried granular, powder or oily ligniteproduct is unsuitable for storage outdoors (for example, on the groundin windrows) exposed to atmospheric condition. Most require specialstorage and handling facilities due to their inherent explosibility orrisk of spontaneous combustion due to high fines content, orenvironmental concerns with oily products in contact with the ground, ortheir potential for degradation under outdoor storage.

During the 1980's studies were carried out in Victoria's LaTrobe Valleyin an attempt to establish an industrial process for solar drying oflignite to create a high quality product for carbonisation, and as apossible replacement for briquettes. As a result of these studies apurpose built pilot plant was built and operated at Hazelwood in theLatrobe Valley for a period of around three years.

The pilot plant was located remotely from the mine so lignite wastransported from the mine to the plant using trucks. Inside the plantthe lignite was passed through a grinding mill for reduction to smallparticles, and added to water in a slurry mixer. The resulting slurrywas then pumped into specialised concrete lined above-ground ponds whereit was left to dry under the sun. Once the material in the pondage haddried sufficiently it would be harvested and broken into storable lumpsof hard lignite similar in characteristics to briquettes. The pilotplant produced approximately 2000 tonnes per annum of product.

However, the pilot plant revealed a number of drawbacks that rendered itimpractical to scale up to a full sized plant.

The mass of handling equipment, the need to transport lignite from themine leads to significant cost and efficiency questions. The need forpurpose built ponds to contain the slurry for drying also added to boththe fixed cost and committed large areas of land for this purpose. Thebases of these ponds needed to be built of a non contaminating materialso that no non burnable, abrasive or other contaminant material couldenter the slurry, and that harvesting was performed easily.

Another problem encountered was the fact that only three harvests peryear were achieved. In order to provide commercially viable quantitiesof product a full sized plant would have had to either provide fiveharvest per year or alternatively, the pondage area would have had to besignificantly increased. The capital cost of a full sized, purpose-builtplant and the need for large scale drying ponds was significant andrendered the process commercially unviable. Furthermore, the cost oftransporting the coal from the mine bench to a remotely locatedpurpose-built plant also imposed significant costs.

Accordingly, there is an ongoing need for a cost effective technologicaland economically viable solution to production of value added ligniteproducts.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing a ligniteproduct comprising the use of a mobile device to dig lignite from aseam, grind the lignite and form a slurry for transportation to a locusfor drying, such as the slot from which the lignite was dug. Control ofthe particle size during digging and grinding together with carefulchoice of any useful additives concomitantly provides control of thecharacteristics of the slurry and the product.

Typically the mobile device consists of a single unit, or alternativelytwo or more mobile units that are integrated or contiguous.

In a particularly preferred embodiment, the present invention provides amethod for producing a lignite product, the method comprising the stepsof,

-   (a) digging lignite from a lignite seam, the average particle size    of the lignite dug from the seam being less than 100 mm,-   (b) grinding and, optionally, screening to reduce the average    particle size of the lignite to less than 10 mm,-   (c) forming a slurry by adding water to the lignite particles and    further grinding to reduce the average particle size,-   (d) transporting the slurry to a locus for drying, and-   (e) drying the slurry to form the lignite product, wherein steps (a)    to (c) are carried out by a mobile device.

In a particularly preferred embodiment, the ground lignite is acidwashed between steps (b) and (c) to reduce ash and/or other contaminantsin the product.

In a further preferred embodiment, the slurried lignite is alkalitreated during step (c) to modify the characteristics of the slurry andthe resulting product.

The present invention further provides a mobile device suitable forcarrying out the method of the present invention.

In one preferred embodiment the mobile device consists of a single unit.In another preferred embodiment the mobile device consists of two ormore mobile units that are integrated or contiguous. Each unit maycomprise wheels, tracks, skis or the like for traversing a surface suchas the surface of a mine bench. The mobile device, or one or more of theunits comprising the mobile device may be capable of locomotion. One ormore of the units comprising the mobile device may be capable of beingpulled along by a vehicle or machine capable of self locomotion.

It may be necessary to have large hose reels included with the mobiledevice to supply sufficient volumes of water to form the slurry.Suitable commercial hoses will be readily available and known to thoseskilled in the art.

The present invention further provides a method for producing a ligniteproduct, the method comprising the steps of,

-   (a) digging lignite from a lignite seam to create a slot, the    average particle size of the lignite dug from the slot being less    than 100 mm,-   (b) grinding and optionally screening to further reduce the average    particle size of the lignite to less than 10 mm,-   (c) forming a slurry by adding water to the lignite particles and    further grinding to reduce the average particle size,-   (d) transporting the slurry into the slot formed by digging, and-   (e) drying the slurry to form the lignite product,    wherein steps (a) to (d) are carried out by a mobile device,    and wherein the average size of the lignite particles in step (c) is    proportionally related to the distance the slurry is transported in    step (d) by the mobile device.

The slot dug out of the lignite seam is typically 10 to 40 cms in depthand of any convenient length and width. In this embodiment the sundrives the moisture out of the slurry so that the slot contains a long,continuous length of hard lignite product. The hard lignite producttypically undergoes a cracking process due to shrinkage as it dries,causing the product to break up into pieces that can be readilycollected. Alternatively, or in addition, the product can be broken upby manual or automated means.

Typically the mobile device is capable of carrying out the digging,grinding, forming a slurry and casting. It may also be used forreclaiming the cast, hard lignite product or pieces of product once itis dry. For example, typically the mobile device has a cutter head withintegrated grinding ability. In a further embodiment it may have aharvesting head.

Grinding

Step (b) of the process of the present invention may include one or moregrinding steps either integral to the digging step (a) or contiguous toit, or both. For example, The digging device may arrange its cutter headand cutter box to achieve a grinding step within the cutterbox or it maypass the dug material to a single grinding device, or alternatively,multiple different grinding devices.

The lignite particles may be ground any suitable milling or grindingapparatus known to those skilled in the technology such as hammer mill,attrition mills, stirred ball mill, powder grinders, Dispax mill, rollermill or fine grinders. The optimal particle size and size range willdepend on the particle sizes appropriate to the input requirements ofsubsequent stages of the process and the final product characteristics.Screening

Step (b) may optionally include screening the lignite. Typically one ormore screens will be used. The screens may be rotary screens, flatscreens or inclined screens although other suitable types of screenswill be readily apparent to the person skilled in the art. Materialscreened off, may be redirected to other processes. For example, flakematerial is typically of 20-50 mm in size and is readily screened offand sold as a stand alone product to customers who can use it in theirprocesses. In a particularly preferred embodiment of the process of thepresent invention, multiple screens are used to screen off multiplestand alone products that can be used in other processes, includingother drying processes. In a further embodiment, the larger materialsare redirected in front of the present invention and are re-dug andpassed through steps a and b

Slurrying and Fine Grinding

Step (c) of the process of the present invention may include multiplegrinding steps. For example, water may be added to the lignite and themixture ground once, before more water is added and further grindingcarried out. This may be repeated as many times as necessary until theparticle size is reduced to the desired specification. Typically a twostage fine wet grind is optimal. A single grinding device, oralternatively, multiple different grinding devices may be utilised.

The lignite particles may be ground by any suitable milling or grindingapparatus known to those skilled in the technology such as attritionmills, stirred ball mills, powder grinders, Dispax mill or finegrinders. The optimal particle size and size range will depend on thedistance the slurry is to be transported to reach the locus at which itis to be dried, the manner in which it is transported (e.g. auguring orpumping), and also the desired characteristics of the dry end-product.

Slurry

Preferably the slurry comprises 15 to 40 wt % lignite, more preferably20 to 30 wt % lignite, and 60 to 85 wt % water.

The slurry consistency will depend upon the amount of water added andany admixture. Other additives or admixtures may include, for example,calcium hydroxide, ammonium hydroxide, coal ash, ash water or otheralkaline material. Alkaline material is typically added to achievebetter grinding performance including lower power consumption, asuitable slurry consistency and desirable final product characteristics.

Preferably the pH of the slurry is between 5.0 and 8.5 and alkalinematerial such as Ca(OH)₂ or other hydroxides may be added to achieve thedesired pH. Control of the slurry pH enables optimisation of the slurrymilling, slurry rheology and lignite product. It has been found that atpH values below 5 the coal colloid spontaneously flocculates and theresultant lignite product tends to be soft, and of low density and highporosity.

Without wishing to be bound by theory it appears that the higher pH hasa significant effect on the slurry product by altering the structure ofthe lignite. Lignite is essentially composed of a bimodal mixture ofpartially degraded plant matter; the larger particles (xyliticfragments, spores, waxes, resins and possibly rod like cell remains) areheld together by a “humic acid” glue. When brown coal is extracted by astrong base the humic glue is removed leaving behind weak conglomeratesof xylitic plant residues. These xylitic fragments are highly porous andhave a large surface area. The humic acids contain many oxygenfunctional groups and in raw coal function as a binder in a similarfashion to that of a colloid which is flocculated by polyelectrolytepolymers. Accordingly, alkali digestion of porous raw coal significantlydisrupts the structure and many of the pores and cavities which havedeveloped during the coalification process are either destroyed orfilled with small particles and humic acid macromolecules.

It will be appreciated that additives may be mixed with the particulatelignite or slurry to assist grinding, initiate digestion and aid slurryformation or pumping.

Slurry stability and consistency may be enhanced by the addition ofvegetable gums or starches.

Transporting the Slurry

The slurry is typically transported using an auger or a slurry pump,such as a centrifugal slurry pump, or positive displacement pump. Whilethe particle size of the lignite is less than 10 mm, in many cases theparticle sizes in the slurry will range across the entire 10 mm. Theparticle size range is one factor that affects the pumpability of theslurry. When the slurry is pumped through a pipeline, optimally theaverage size of the lignite particles in step (c) is proportionallyrelated to the distance the slurry is transported in step (d).

The slurry may require continuous agitation to achieve a flowableproduct for pumping. If the slurry is to be cast into a slot in thelignite created during digging, further agitation and possibly screedingmay be required to create a uniform product.

Drying

The drying may be carried out by any convenient means such as spraydrying, cyclone drying technologies, fluidised bed, application of wasteheat or heat from other low grade heat sources including geothermal orhydrothermal sources, roller drying or solar energy. For example,suitable cyclone drying technology includes the Windhexe® device fromVortex Drying Technology in the USA or a cyclone destruction machineavailable from Australian company DevourX Pty. Ltd which claims 100 t/hrperformance.

In a particularly preferred embodiment the method of the presentinvention utilises solar power, and locus for drying is a slot in thelignite seam created when the lignite being processed was dug from theseam.

In the initial stages of drying the slurry, where bulk water is beingevaporated, the drying rate is equal to that of free water. As morewater is evaporated the drying rate decreases as moisture isprogressively removed from the macropores, capillaries and micropores.

For high slurry moisture contents (≈70%) practically 100% runoff isachieved when solar and air drying is used, however, below a slurrymoisture content of about 35%, harvesting and further drying in windrowsor covered areas may be required. Supplementary heating or airflow mayassist drying.

Lignite Product

The lignite product of the method has a moisture content of less than 40wt % (as compared with 60 to 70 wt % moisture in lignite in situ.)Preferably the lignite product produced by the method of the presentinvention has a moisture content of 15 to 35 wt % water.

The lignite product typically has a Hardgrove Index values from 30 to70, preferably between 40 and 60, more preferably 45. The HardgroveIndex gives an indication of the grindability of a material, the lowerthe index the harder and more difficult the material is to grind. (Forcomparison, the typical Hardgrove Index values for briquettes vary from40 to 70 and for brown coal vary from 70 to 190.)

The lignite product also has a needle hardness greater than the typicalvalues for brown coal. Typically the needle hardness for the ligniteproduct is from 7.0 to 15.0 kg/mm² and the specific gravity is from0.900 to 1.3 g/cc. The needle hardness results are considerably lowerthan those of briquettes (25-50 kg/mm²).

The product naturally forms into hard dense lumpy lignite product bycracking formation during the drying process. The product lumps resistweathering and therefore assist in reducing nuisance dust and fire riskon the mine levels. The product may be harvested directly from thecasting pit on the mine levels, and may be placed in windrows forfurther air drying, or used directly in the power generation facility ortaken off for use in other process plants. Harvesting can be carried outby the machine of the present invention or by auxiliary plant. Since theproduct has had much of the moisture removed from the lignite, an energydense product (MJ/kg) is the available for transport from the mine inplace of low energy density lignite.

The lignite seam for this processing would typically be located on amine bench or mine floor. As previously mentioned, lignite mines aretypically located adjacent electric power generation facilities.Accordingly, in a preferred embodiment the product of the method of thepresent invention would typically be fed directly to the fuel system ofthe combustion chamber of the power generation facility or it may betransported to act as a feedstock for other processing plants. Coal in aclean dry state can be turned into a range of other products.

While the number of harvests per year will be variable due to seasonalvariations, it is expected that between three to five harvests will beachievable, based on previous experience, and the availability of themine's coal level for the digging and slurrying the coal, casting, anddrying the product.

Advantages

The method of the present invention has a number of significantadvantages as compared with the prior art. For example, the method ofthe present invention eliminates the need for large size, speciallyprepared drying ponds as used by the prior art. Furthermore, the presentinvention avoids the need for a specialised fixed processing plant,steam or heat raising plant and virtually eliminates cartage andhandling to the processing plant. Furthermore it creates an energy denseproduct for storage or transport directly from the mine, and as theproduct lumps resist weathering and cover the coal surface on a level,they are expected to assist in reducing nuisance dust and fire risk onthe mine levels.

If the product of the method of the present invention is fed directly tothe fuel system of the combustion chamber of the power generationfacility adjacent the location where the lignite has been mined, thiswould improve the efficiency/greenhouse intensity of the powergeneration.

DRAWINGS

Various embodiments/aspects of the invention will now be described withreference to the following drawings in which,

FIG. 1 is a drawing of a typical process of the prior art.

FIG. 2 is a drawing of a preferred embodiment of the present invention.

FIG. 1 is a flowchart setting out the typical stages in a process of theprior art for making a dry lignite product. The steps can be summarisedas follows:

-   -   Dig lignite from an open cut mine bench, the dug lignite having        a maximum particle size of 250 mm.    -   Transport the dug lignite from the mine bench to a plant in 20        tonne trucks    -   Store until required for processing.    -   Transport the lignite from storage to a grinding mill by        conveyor belt.    -   Crush the lignite in the grinding mill to a particle size of 10        mm.    -   Create slurry by adding water.    -   Grind and screen the lignite in the slurry to a maximum particle        size of 7.5 micron and pump the slurry into purpose built cement        ponds.    -   Sun-dry the slurry in the cement ponds.    -   Harvest is typically by wheeled or tracked loader or excavator        and loaded into truck or onto conveyors which ever is available        from the operation.    -   Transport the harvested product to storage area.    -   Store the product in windrows.

FIG. 2 is a flowchart setting out the typical stages in one embodimentof a process according to the present invention for making a dry ligniteproduct. The steps can be summarised as follows:

-   -   Dig the lignite from an open cut mine bench, the dug lignite        having a particle size diameter less than 100 mm.    -   Grind and screen the dug lignite to less than 10 mm.    -   Mix the lignite with water and any necessary admixtures to form        a slurry and grind the particles in the slurry as required to        provide the desired product characteristics.    -   Transport the slurry into the slot formed when the lignite was        dug and leave the slurry to dry under the sun, the resultant        product having a typical moisture content of 20 to 35 wt %.    -   Harvest the lignite product.    -   Transport the harvested product to storage area.    -   Store the harvested product in windrows.    -   Or use directly from the slot if required.

The word ‘comprising’ and forms of the word ‘comprising’ as used in thisdescription and in the claims does not limit the invention claimed toexclude any variants or additions. Modifications and improvements to theinvention will be readily apparent to those skilled in the art. Suchmodifications and improvements are intended to be within the scope ofthis invention.

1. A method for producing a lignite product comprising the steps of; (a)digging lignite from a lignite seam, (b) grinding the lignite, (c)forming a slurry comprising the lignite, (d) transporting the slurry toa locus for drying, and (e) drying the slurry to form a lignite product,wherein steps (a) to (c) are carried out using a mobile device.
 2. Amethod according to claim 1 wherein in step (a) digging the lignitecreates a slot in the lignite seam and in step (d) the slurry istransported into the slot.
 3. A method according to claim 1 wherein theground lignite of step (b) is acid washed prior to step (c).
 4. A methodaccording to claim 1 wherein step (b) includes multiple grinding stepsusing one or more grinding devices.
 5. A method according to claim 1wherein step (b) includes screening the lignite.
 6. A method accordingto claim 5 wherein step (b) includes the use of multiple screens.
 7. Amethod according to claim 5 wherein material screened off is used inother processes.
 8. A method according to claim 1 wherein the slurrycomprises between 15 and 40 wt % lignite and 60 to 85 wt % water.
 9. Amethod according to claim 1 wherein the slurry of step (c) is alkalitreated.
 10. A method according to claim 1 wherein the slurry has a pHof between 5.0 and 8.5.
 11. A method according to claim 1 wherein theaverage size of the lignite particles in step (c) is proportionallyrelated to the distance the slurry is transported in step (d).
 12. Amethod according to claim 1 wherein the drying is carried out by amethod chosen from the group comprising, spray drying, cyclone drying,thermal drying, air drying, roller drying or solar drying orcombinations thereof.
 13. A method according to claim 1 wherein thelignite product of step (e) has a Hardgrove Index of between 30 and 70.14. A method according to claim 1 wherein the lignite product of step(e) has a needle hardness of from 7.0 to 15.0 kg/mm² and specificgravity of from 0.900 to 1.3 g/cc.
 15. A method according to claim 1wherein the method comprises the steps of, (a) digging lignite from alignite seam to create a slot, the average particle size of the lignitedug from the seam being less than 100 mm, (b) grinding to reduce theaverage particle size of the lignite to less than 10 mm, (c) forming aslurry by adding water to the lignite particles and further grinding toreduce the average particle size, (d) transporting the slurry into theslot formed by digging, and (e) drying the slurry to form the ligniteproduct, wherein steps (a) to (d) are carried out by a mobile device,and wherein the average size of the lignite particles in step (c) isproportionally related to the distance the slurry is transported in step(d) by the mobile device. 16.-26. (canceled)
 27. A mobile devicesuitable for use in the method of claim
 1. 28. A mobile device accordingto claim 27 comprising a single unit.
 29. A mobile device according toclaim 28 comprising at least two units, wherein the units areintegrated.
 30. A mobile device according to claim 27 comprising acutter head for digging lignite from a lignite seam, at least onegrinder for grinding lignite and a slurry mixer for forming an aqueouslignite slurry.
 31. A mobile device according to claim 27 comprising oneor more screens for screening the lignite.
 32. A mobile device accordingto claim 27 that further comprises a harvesting head.